The present invention relates generally to magnetic resonance (MR) imaging and, more particularly, to an RF coil embedded with homogeneity enhancing material such that an improved MR image of a subject may be reconstructed.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Magnetic resonance imaging (MRI) is generally predicated on the excitation of hydrogen atoms within the tissue. Human tissue typically has high water content and hydrogen is plentiful in water. Therefore, MR imaging based on scanning for hydrogen is typically preferred for diagnostic purposes. It is well known that in hydrogen MR imaging, tissues that have little or no hydrogen produce very little or no signal. Conversely, tissues or fatty regions with high hydrogen content are highly emissive and provide a strong MR signal. However, if the hydrogen content of the tissue is exceptionally high relative to nearby tissues, the resultant signal may overwhelm and mask the details of nearby tissues with less hydrogen content. This is particularly problematic when the lower hydrogen content tissues are diagnostically significant in the MR imaging session.
A number of imaging techniques have been developed to alleviate the aforementioned problem and are designed to separate the signals that are emitted from the various tissues subject to the MR imaging process. These methods include nullification of signals from certain tissues. One such method commonly referred to as “fat saturation” requires that the entire tissue be subjected to an off-resonance specific saturation pulse (off by 3.3 ppm). The saturation pulse “deactivates” the lipid portions of the tissue such that useful signals are not emitted from fat when the imaging portion of the data acquisition signal is applied.
For fat saturation to be effective, the homogeneity must be precise. If not, non-lipid tissues may be off resonance by several ppm and inadvertently “deactivated” by the off-resonance specific saturation pulse resulting in an incomplete and, potentially, unusable image. Additionally, MR imaging of the neck and shoulder regions of a patient is particularly susceptible to ineffective fat saturation as these regions of the patient often have more field inhomogeneity due to the geometry of the neck and/or shoulders. One method to address the limitations or imprecision of fat saturation in the neck and shoulder area includes the placement of a bag of water or non-protonated fluid around these regions of the patient. Water, which is highly protonated, however can create a bright signal in the resultant MR image, a disadvantage discussed above. Therefore, implementation of bags or containers of non-protonated fluids, such as fluorocarbons, is preferred.
Fluorocarbons have magnetic susceptibility properties similar to that of human tissue. They have been found to be highly effective in correcting the field inhomogeneity, thereby improving the ability to saturate only fat tissue. Specifically, hydrogen-depleted fluorocarbons have magnetic susceptibility properties similar to that of human tissue and since they have low hydrogen content, they do not contribute any signal to the MR image.
Specifically, bags of fluorocarboneceous material or “sat pads” when properly used can reduce the influence of the human body on the magnetic flux. The magnetic flux can be thought of as traveling down the magnet bore in the Z direction. When a subject is in the bore of an MR system, the tissue water content is diamagnetic and hence has an influence on the magnetic flux. Looking inwardly through the magnet bore, the cross section of the subject typically expands and contracts depending upon the anatomical location. This variation in cross-section has a small but significant influence on the magnetic flux. Wherever the magnetic flux direction and strength are altered by geometric changes of the subject, the homogeneity of the magnetic field decreases in those regions. For example, as will be discussed with respect to FIG. 7, coil assemblies are commonly constructed to include a protrusion or “bump” that acts a neck rest for a patient. Because it is desirable to minimize the distance between the patient and the RF coil, the RF coil elements are typically placed to match the contour of the neck rest. As a result, air volumes are formed in the coil assembly that can negatively affect magnetic flux. That is, magnetic flux traveling through the patient will encounter the air volumes and react differently from the interactions with the tissue water of the patient. As a result, the magnetic flux will change direction and negatively affect homogeneity. As a result, if the cross section variations could be diminished, the homogeneity would improve.
An alternative but less desirable implementation uses doped water with extremely rapid signal decay so as not to produce any significant signal during a typical NMR measurement. At least two disadvantages of doped water are its permittivity and electrical conductivity. The RF performance and safety of doped water are also problematic.
Notwithstanding the advantages achieved by sat pads, they can be problematic when acquiring imaging data from particular regions of the patient. For example and as shown in FIG. 7, when acquiring data from the neck region, a patient 1 is placed on a table 2 having an RF coil assembly and that includes a protrusion 3 extending therefrom. The protrusion 3 serves as support for the neck 4 of the patient 1, but also houses an RF coil (not shown). By enclosing the coil within the protrusion 3, the coil may be positioned closer to the neck which improves reception signal strength and, ultimately, results in a better diagnostic image. Placing a sat pad 5 on an upper end 6 of the protrusion 3 to reduce changes in cross-section between the torso and the neck of the patient causes the curvature 7 of the neck 4 to be exaggerated and thereby defeats the intent of minimizing contour changes. Additionally, placement of the sat pad 5 increases the distance d between the RF coil and the neck which can decrease the signal strength detected by the coil.
It would therefore be desirable to have a system and method capable of improving fat saturation with a homogeneity enhancing material that does not result in an exaggeration of anatomical contours of the patient or increases the distance between the patient and the RF coil.